A post-processing apparatus that performs post-processing on sheets transported from an image-forming apparatus is known. The post-processing apparatus includes a processing tray and a standby tray. In the processing tray, post-processing is performed. The standby tray is provided above the processing tray. During the post-processing performed on sheets in the processing tray, the standby tray temporarily retains subsequent sheets. When the processing tray becomes empty, the standby tray drops the retained sheets toward the processing tray. Incidentally, the post-processing apparatus aligns multiples sheets and then performs stapling processing as one post-processing thereon. In order to accurately perform the stapling processing, it is necessary to improve accuracy of sheet alignment as a preceding process. However, there has been a case where the accuracy of sheet alignment is difficult to sufficiently increase depending on a transport state of the sheets.